What's particularly fascinating is how the coastlines and transportation conduits — including those on the ocean — are so clearly demarcated. It's also interesting to see those regions that are either low density, inaccessible — or even off the grid. There may also be selectional effects at play here as the maps were compiled exclusively from geotags; population density does not necessarily correlate to tweet density. Nor is twitter used to the same degree in different parts of the world.